Fumonisins A1, A2, B1, B2 and B3 are a series of mycotoxins produced by strains of Fusarium moniliforme. Fumonisins are hydroxylated long-chain alkylamines esterified with propanetricarboxylic acid moieties that represent approximately half the mol. wt of the toxins. The A-series fumonisins are N-acetylated, whereas the B series contains free amino groups. Hydrolytic removal of the propanetricarboxylic acid moieties from fumonisins B1 and B2 yields the corresponding aminopentols, AP1 and AP2, respectively. These compounds were tested for toxicity on widely differing bioassay systems, representing plant and animal systems. The plant bioassay system employed jimsonweed (Datura stramonium L.) leaves and leaf discs in which toxicity was detected as electrolyte leakage, photobleaching and quantitation of chlorophyll reduction. The animal bioassay system employed cultured mammalian cell lines in which toxicity was detected as inhibition of cell proliferation. Fumonisins B1, B2 and B3 at 50 micrograms/ml or less were effective toxins after exposure periods greater than 24 hr in all plant and animal bioassay systems examined, except 3T3 mouse fibroblasts, whereas fumonisins A1 and A2 exhibited little or no activity. However, the hydrolytic degradation products AP1 and AP2 exhibited toxicity similar to or greater than B-series fumonisins in all test systems, including substantial toxicity to 3T3 mouse fibroblasts.